The invention relates to a sound wave generator for therapeutic applications such as the contactless smashing of a concrement situated in the body of a living being.
A device of the above-mentioned type is described in the European Patent Document EP 0 275 427. In front of a flat coil, an insulating foil is arranged and in front of the insulating foil, a diaphragm is arranged which is made of an electrically conductive material. By the feeding of a discharge current to the flat coil, that is, by electrical primary energy, the diaphragm is abruptly moved away from the flat coil and generates plane waves in a liquid medium bordering on the diaphragm which propagate perpendicularly to the diaphragm surface and are focussed by means of an arrangement of acoustic focussing lenses.
The German patent Document DE 37 39 390 A1 describes a shock wave source with a flat coil and a diaphragm which is insulated with respect to it, the flat coil consisting of two coil sections which can be activated separately and/or jointly.
In the German Patent Document DE 33 08 637 A1, an ultrasonic apparatus is described which comprises several concentrically arranged ring-shaped ultrasonic emitters, each ultrasonic emitter having the same area.
In the German Patent Document DE 34 17 985 A1, a shock wave generator is described by means of which a focal point can be generated with a clearly increased propagation in the axial direction.
For different applications, for example, for kidney or gallbladder lithotrity, different requirements exist with respect to the focus geometry and the peak pressure.
Because of the problems concerning damage to the tissue, a laterally wider focus (FWHM (Full Width at Half Maximum)=6-10 mm) with peak pressures of approximately 500 bar is desirable. Such values are typically generated by means of sound wave generators of small aperture angles (50.degree.-60.degree.). The comparatively long axial focus (50-100 mm) may be accepted in the case of kidney lithotrity.
For gallbladder lithotrity, on the other hand, a laterally narrower focus (FWHM=3-5 mm) with peak pressures of approximately 800 bar is desirable, in which case the axial half-width value should be at approximately 40 mm, since sensitive structures (liver, lungs, kidneys, spinal column) are situated in front of and behind the gallbladder. Such values are typically achieved by means of shock wave sources of high aperture angles (80.degree.-90.degree.).
It is an object of the invention to provide a sound wave generator of the initially mentioned type which can be adapted to the different applications.
According to the invention, this object is achieved by providing a sound wave generator with a variable effective aperture angle according to the electromagnetic principle for therapeutic applications and particularly for the contactless smashing of a concrement situated in the body of a living being, comprising:
a flat coil arrangement, and
a metallic diaphragm insulated with respect to the flat coil arrangement,
wherein the flat coil arrangement includes an inner coil section, a central coil section concentrically surrounding the inner coil section, and an outer coil section concentrically surrounding the central coil section,
and wherein the electric circuit control apparatus includes means operable to selectively actuate respective different pairs of said coil sections to thereby accommodate use of the sound wave generator with respective different effective aperture angles.
By means of the invention, it is achieved that, by changing the effective aperture angle, the sound wave generator can by optimally used for different therapeutic applications, such as kidney and gallbladder lithotrity. When the plane sound field has a width of D, and the focussing lens system has a focal distance of f, the effective aperture angle is the value EQU 2.multidot.arctan (.sup.D/.sub.2f).
This corresponds to the definition of the aperture angle which is used in optics where the adjective "effective" indicates that a sound-wave-free space may exist between the peripheral rays of the sound field, as in FIG. 2b.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.